1. Field of the Invention
The present invention relates to an improved pin junction photovoltaic element which exhibits an improved photoelectric conversion efficiency and which can be provided at a reduced cost. More particularly, the present invention relates to an improved pin junction photovoltaic element provided with a specific i-type semiconductor layer with a plurality of regions having different graded band gaps which exhibits an improved photoelectric conversion efficiency and which can be provided at a reduced cost.
2. Related Background Art
In recent years, there has been an increased societal demand for early realization of a power generation system capable of providing clean energy without causing CO.sub.2 buildup as in the case of thermal power generation.
There have been made various proposals which are expected to meet such societal demand. Of those proposals, solar cells are expected to be a future power generation source since they supply electric power without causing the problem above mentioned.
There are a number of solar cells which have been put into practical use. Among those solar cells, there is a solar cell made of a single crystal silicon material (which is commonly called the "single crystal silicon solar cell"). This single crystal silicon solar cell has been found to be highly reliable and high in photoelectric conversion efficiency. This single crystal silicon solar cell is produced in a manner similar to the semiconductor wafer process of producing an integrated circuit. That is, a single crystal silicon ingot containing a minute amount of a dopant (i.e. a valence electron controlling agent) of p-type or n-type is first prepared by a crystal growth method, a silicon wafer is cut from the ingot, and a p-n junction is formed in the silicon wafer thus obtained by diffusing a different dopant therein. In the single crystal silicon solar cell, light such as sunlight is absorbed, the absorbed light generates excited photocarriers and those photocarriers migrate under the action of an internal electric field of the p-n junction, whereby a photoelectromotive force is generated. However, the single crystal silicon solar cell is costly since it is produced by a semiconductor wafer process as above mentioned. In addition to this, there is a disadvantage in the single crystal silicon solar cell in that there is a limit to the area of a silicon wafer that can be produced because of the requirement for growing a single crystal and it is therefore extremely difficult to make it in a large area. Further, in order for the single crystal silicon solar cell to be employed in a module usable as a power source outdoors, it is necessary to provide the cell with protective equipment. In consequence, the resulting single crystal silicon solar cell module unavoidably becomes relatively costly in comparison with oil fired power generation in terms of production cost of a unit of electricity.
In any case, in order to make solar cells usable as practical power sources, it is an essential that a large area solar cell be industrially mass-produced at a reduced cost. This requirement is not met by the above single crystal silicon solar cell.
Thus, various studies have been made on the constituent semiconductor films from various viewpoints such as reproducibility, productivity, production cost, etc.
So far there have been proposed various non-single crystal semiconductor films such as tetrahedral non-single crystal semiconductor films of amorphous silicon (a-Si), amorphous silicon germanium (a-SiGe), amorphous silicon carbide (a-SiC), etc., compound semiconductor films containing elements of groups II and VI of the periodic table such as CdS, CdTe, Cu.sub.2 S, ZnSe and ZnTe semiconductor films, and other compound semiconductor films containing elements of groups I, III, and VI of the periodic table.
Among these semiconductor films, the tetrahedral non-single crystal semiconductor films (that is, amorphous silicon semiconductor films) are expected to be the most promising in view of their various advantages, such as, for example, that a large semiconductor film capable of being the constituent of a solar cell can be relatively easily formed, that this semiconductor film can be thinned as desired and formed on a selected substrate, and that a large area solar cell can be produced on an industrial scale, and thus it is possible to provide a solar cell at reasonable cost.
Now, as such a solar cell, there have been proposed a number of solar cells in which non-single crystal semiconductor films (amorphous semiconductor films) are used (these solar cells will be hereinafter referred to as non-single crystal solar cells or amorphous solar cells). Non-single crystal solar cells which are presently known are typically pin junction type, MIS type (comprising metal layer/insulating layer/semiconductor layer) or heterojunction type. Of these, solar cells of pin junction type (hereinafter referred to as "pin junction solar cells" or "pin junction photovoltaic elements") are of greatest interest. The pin junction solar cell typically comprises a metallic electrode layer, a photoelectromotive force generating layer and another electrode disposed in this order on a substrate, in which said photoelectromotive force generating layer comprises a multilayered body comprising an n-type semiconductor layer, an i-type semiconductor layer, and a p-type semiconductor layer.
There is, however, a disadvantage of such a non-single crystal solar cell in that its photoelectric conversion efficiency is inferior to that of the foregoing single crystal solar cell. In view of this, various studies have been made in order to improve the photoelectric conversion efficiency of such a non-single crystal solar cell.
Particularly, for amorphous silicon (a-Si) which is the amorphous semiconductor most widely used at the present time, its energy band width is about 1.7 eV or more, and because of this, light of 750 nm or more in wavelength is barely absorbed by the amorphous silicon.
Therefore, long wavelength energy components of the sunlight spectrum are not efficiently utilized.
In order to eliminate this problem, studies have been made on other silicon-containing amorphous materials comprising a-Si incorporating appropriate atoms such as germanium atoms (Ge), tin atoms (Sn) or lead atoms (Pb) which have a band gap narrower than that of a-Si (amorphous silicon). As for these silicon-containing amorphous materials, there is an advantage that their band gap can be properly adjusted by controlling the amount of such atoms incorporated thereinto.
There has been proposed a pin junction solar cell (which is a so-called single cell type pin junction solar cell) in which such narrow band gap silicon-containing amorphous material is used as the i-type semiconductor layer. Further, there has been proposed a multicell-stacked solar cell comprising a pin junction solar cell element having an i-type semiconductor layer formed of such narrow band gap silicon-containing amorphous material and another pin junction solar cell element having an i-type semiconductor layer formed of a-Si. It is generally believed that the latter multicells-stacked solar cell provides a photoelectric conversion efficiency greater than that provided by the former single cell type solar cell.
However, there are disadvantages for the foregoing silicon-containing amorphous materials in that they are inferior to a-Si in terms of the combined states of the atoms; films of them are liable to easily deteriorate in comparison with the a-Si; and they are also inferior to the a-Si in terms of the product .mu..tau. of an electron and a hole (that is the product of a carrier's mobility and a carrier's mean life time).
Now, U.S. Pat. No. 4,816,082 (hereinafter referred to as "Literature 1") proposes a pin junction solar cell having an i-type semiconductor layer comprising an amorphous alloy in which the band gap of the i-type semiconductor layer is graded such that the band gap of each of the regions in contact with the p-type semiconductor layer and the n-type semiconductor layer is widened and is a minimum at a central region. This proposed pin junction solar cell aims at providing an improved photoelectric conversion efficiency by varying the band gap of the i-type semiconductor layer depending on the position thereof n the thickness direction.
Besides the above proposal, U.S. Pat. No. 4,542,256 (hereinafter referred to as "Literature 2") proposes a pin junction photovoltaic cell (namely, pin junction solar cell) which is aimed at providing an improved photoelectric conversion efficiency by forming the n-type semiconductor layer or the p-type semiconductor layer of a material having a band gap wider than that of the constituent material of the i-type semiconductor layer and disposing a layer (that is a so-called intermediate layer) having a continuously graded band gap not only at the interface between the p-type semiconductor layer and the i-type semiconductor layer but also at the interface between the n-type semiconductor layer and the i-type semiconductor layer.
However, neither of these proposals is sufficient in terms of providing high enough photoelectric conversion efficiency in a pin junction solar cell.